The invention relates to an optical fiber comprising an internal coating and a colored coating, further called colored optical fiber, and to an optical fiber ribbon comprising a plurality of said colored optical fibers.
Optical glass fibers are generally coated with two superposed radiation-cured coating layers, which together form the so-called primary coating or primary coating system. The coating layer (more briefly “coating”) which is in direct contact with the glass is called the inner primary coating and the overlaying coating, which is on the exposed surface of the coated fiber, is called the outer primary coating. The inner primary coating may also be called the primary coating; then, the outer primary coating is called the secondary coating. Both definitions are used interchangeably.
The inner primary coating is usually a relatively soft material while the outer primary coating is a relatively harder material. The primary coating system is designed to provide environmental protection to the glass fiber and resistance, inter alia, to the well-known phenomenon of microbending, which can lead to attenuation of the signal transmission capability of the fiber and is therefore undesirable. In addition, the primary coating system is designed to provide the desired resistance to physical handling forces, such as those encountered when the fiber is submitted to cabling operations.
In general, the primary coating system is applied onto the optical fiber during the drawing manufacturing process of the optical fiber.
In telecommunications applications of optical fibers, multiple individual strands of coated fiber can be packaged into larger structures such as ribbons and cables, to maximize efficiency. However, after ribboning and cabling of fiber, the individual strands of fiber must be readily distinguishable from each other so they can be accurately identified during, for example, installation and repair. Cable geometry and/or color coding can be used to distinguish and identify individual fibers in a complex cable.
Although several methods can be used to color code fiber, color coding can be done advantageously with either a thin colored layer (about 10 microns or less), also called an ink composition, which is placed over the primary coated fiber before cabling and/or ribboning of the same or by applying a colored outer primary coating onto the inner primary coating.
Typically, the application of the colored outer primary coating onto the inner primary coating takes place during the drawing process of the optical fiber. On the other side, the application of a colored layer onto the primary coated optical fiber generally takes place on a separate manufacturing line, after the primary coated optical fiber has been produced.
For the sake of conciseness, in the following of the present specification the term “internal coating” will indicate a coating disposed to surround the glass portion of the optical fiber, thus comprising either an “inner primary coating” or a “primary coating system” (i.e. comprised of an inner and an outer primary coating). Said internal coating is then in turn coated with a colored coating. The terms “colored coating composition”, “colored layer”, “ink layer” and “ink composition” are used interchangeably throughout the specification.
Tape-like optical fiber ribbons are prepared by embedding at least two individual color coded fibers in a supporting matrix material which, like the inner and outer primary coatings, is also radiation-curable to maximize production speed. Optical fiber ribbons may comprise, for instance, 4 to 12 colored fibers. The matrix material can encase the color coded optical glass fiber or the matrix material can edge-bond the glass fibers together. Cure of the matrix material occurs during the ribboning stage after the fibers have been color-coded by applying a colored coating. Hence, in a ribbon design, the ink layer resides between the ribbon's matrix material and the fibers' outer primary coating.
This means that the ink layer's interfacial characteristics (e.g., surface energy, adhesion) must be carefully controlled to function properly with both matrix material and outer primary coating in the ribbon structure. In particular, the ability of a cured matrix material to be suitably stripped off the ink layer (break-out) is an important technical consideration. Ribbon break-out is generally carried out by a mechanical force, although chemical softening of the matrix with use of solvents is also known.
Optical fiber color coding can be based on up to 12 or more colors. Although optical fiber inks were originally solvent-based or thermosetting inks, in more recent times, radiation-curable inks have been used to increase the speed of the inking process. In these ink compositions, pigment is dispersed in a radiation-curable carrier or base composition.
As the demand for coated optical glass fibers has increased, manufacturers must respond by adding more fiber drawing production lines and by attempting to increase the linear line speeds of the existing fiber drawing/coloring production lines. In the latter case, one factor which will determine the upper limit for the line speed will be the curing rate characteristics of the radiation-curable ink composition, for a given radiation source and intensity.
If the line speed is increased to the extent that cure rate time requirements of the radiation curable ink composition are not provided, the radiation curable ink composition will not have received a sufficient amount of radiation to cause complete cure, or cross-linking, of the radiation-curable ink composition.
The production linear line speed is generally inversely related to the amount of radiation striking the optical glass fiber. That is, as the production line speed is increased, the amount of radiation exposure to the radiation-curable ink composition during the production process will necessarily decrease for a given radiation source. Incomplete cure of the radiation-curable ink composition is undesirable and must be avoided because then the desired properties of the ink coating may not be achieved and/or the incompletely cured ink coating may retain tackiness (giving problems in subsequent handling) or a malodorous odor may be present, and there may also be an undesirable increase of extractable components in the supposedly-cured ink coating.
In general, radiation-curable ink coating compositions cure at a significantly slower rate than radiation-curable outer primary coating compositions.
It is believed that the pigments present in ink compositions contribute to the slower cure speed of ink coatings. Thus, there is a need for improving the cure speed of the ink.
While the ink composition must have a very fast cure speed to ensure complete cure of the ink coating on the high speed drawing/coloring lines, the increase in cure speed should not come at the expense of other important properties of the ink coating, such as that providing suitable break-out performance. Break-out performance is the ability of the cured ink coating to separate from the matrix material without separating the ink layer from the outer primary coating, to provide an easy access to the individual coated optical glass fibers contained within the ribbon assembly, for instance during cabling/connection operations of the optical fibers.
Therefore, a radiation-curable ink composition should preferably exhibit adaptable adhesion properties to provide an adhesion between the outer primary coating and the ink coating that is greater than the adhesion between the ink coating and the matrix material to provide easy fiber access.
International Patent application Publication No. WO 98/50317 discloses a ribbon assembly comprising a colored optical fiber, wherein the colored coating of said optical fiber is formed from a radiation curable system which contains a mixture of oligomers, monomers and at least one photoinitiator, selected in such a way as to provide a level of adhesion between the ink coating and the matrix material which is less than the level of adhesion between said ink coating and the underlying inner coating of the optical fiber.
Patent application EP-A-614099 describes the use of a release agent such as a silicon oil or a fluororesin between the bundling layer and the coloring layer. In particular, when substantial amounts of silicone resins are used, incompatibility in the liquid and imperfections in the cured matrix composition may result, which causes attenuation of light.
Published Japanese patent application JP-A-01022976 describes a radiation curable ink composition comprising an alkoxylated bisphenol A diacrylate oligomer, a trifunctional reactive diluent and a homolytic photoinitiator.